Stripline signal processing circuits can be used to implement a variety of analog signal operations on electromagnetic energy propagating within the circuit at radio frequency (RF) and microwave transmission frequencies. Generally stated, a stripline signal processing circuit, as that term is used in this specification, is a circuit that includes one or more transmission media segments of specified lengths and impedance characteristics, which are typically interconnected into a network, and which exhibit a desired frequency response (also called a “transfer function”) that performs a desired signal processing operation on electromagnetic energy propagating through the circuit. The term “microstrip” is commonly used to refer to stripline circuits having two conductors in which the transmission media segments are exposed to one or more dielectric materials on a first side backed by a conducting plane and one or more dielectric materials including air on a second side without a second conducting plane. The term “tri-plate stripline” is commonly used to refer to stripline circuits that include transmission media segments are exposed to one or more dielectric materials on both sides bounded by a conducting plane on each side. In addition, the terms “air microstrip” or “air stripline” are commonly used to refer to stripline circuits in which the transmission media segments are exposed to air on both sides. All of these circuit configurations fall within the class of circuits referred to as “stripline” in this specification.
A stripline circuit often does, but does not necessarily, include one or more lumped (also called “discrete” or “conventional”) electric elements, such as resistors, capacitors and inductors interconnected with the stripline segments within the circuit. These circuits may also, but need not necessarily, include active elements or stages such as active amplifier stages, and non-linear elements such as diodes, transistors, and other conventional circuit elements. In addition, these circuits may also, but need not necessarily, include other types of transmission media segments, such as coaxial cable, tubular waveguide, and so forth, as well as junctions between different types of transmission media. Because the electromagnetic energy is processed by the circuit as the signal propagates through the circuit, these circuits are typically characterized by a network of stripline segments connected between a plurality of input ports and a plurality of output ports, in which a desired signal processing operation is performed on the signal as it propagates from the input ports to the output ports.
Stripline signal processing circuits may be used to implement a wide range of functions, such as signal dividing, signal amplification, signal combining, signal encoding, and so forth. In general, they are typically used to construct relatively simple functions or modules, as described above, which are combined into more complicated structures configured to implement higher-level components, such as beam forming networks, hybrid matrix amplifiers, radio frequency amplifiers, and so forth. These higher-level components, in turn, may be interconnected and controlled to implement a wide range of commercial devices, such as radars for missiles and missile defense, satellite communication systems, wireless telephone base station antennas, Doppler radars, and many others.
Stripline signal processing circuits are typically referred to as “reciprocal” when the transfer function is the same for a signal propagating from the input ports to the output ports as it is for a signal propagating from the output ports to the input ports. Reciprocal signal processing circuits are particularly well suited for use in radar systems that both emit and receive electromagnetic energy through the same transmission path. Orthogonal signal processing circuits are a particularly important class of signal processing circuits that are characterized by a plurality of input ports that are isolated from each other. This allows the signal injected into each input port to be independently controlled without substantial interference from the other input ports. Reciprocal orthogonal circuits, even more specifically, are an important class of signal processing circuits that are well suited to a range of applications using analog amplifiers and beam forming networks. The reciprocity property can be obtained through the reuse of a portion or all of a passive circuit for bi-directional signal flow.
As noted above, an orthogonal circuit includes a number of isolated input ports, and also typically includes a plurality of output ports that each receive a weighted, phase-adjusted combination of the input signals injected into the input ports. That is, the output signal at each output port typically includes a linear combination or “superposition” of input components, in which each input component is an amplitude-weighted, phase-adjusted portion or division of the signal injected into one of the input ports. In other words, the input signals are isolated from each other, and each input signal is divided into a number of weighted and phase-adjusted components that are delivered to the output ports, such that each output port produces an amplitude weighted and phase-adjusted linear combination of the input signals. In addition, the input and output impedances of the orthogonal circuit are typically matched across connecting junctions or ports so that the ideal non-absorbing circuit is theoretically lossless for signal flow between ports. That is, the orthogonal circuit does not absorb or reflect back any of the energy injected into the input ports, but instead divides and delivers all of the input energy to the output ports, where they are combined into amplitude weighted, phase-adjusted linear combinations of the input signals.
Hybrid circuits are a subclass of orthogonal signal processing networks characterized by two inputs, two outputs, and a division of energy from each input port to the output ports. The term “hybrid junction” typically refers to a hybrid circuit in which there is an equal division of power from each input port to the output ports. A “hybrid coupler,” on the other hand, typically refers to a hybrid circuit in which the power division is generally unequal. A hybrid junction in which the phase shift from each input port to the two output ports is zero degrees (0°) and ninety degrees (90°), respectively, is known as a “quadrature” junction or circuit; and a hybrid junction in which the phase shift from each input port to the two output ports is zero (0°) degrees and one-hundred eighty degrees (180°), respectively, is known as a “magic T” junction or circuit. These circuits are also otherwise known as 0°/90° and 0°/180° hybrids. These well known building blocks are usually reciprocal and form the basic building blocks for constructing higher-level orthogonal circuits, which are often referred to as hybrid matrices, Butler matrices, beam forming networks, hybrid matrix amplifiers, RF amplifiers, diplexers, monopulse comparators, and so forth. These building blocks can be used in conjunction with other types of junctions including non-isolating reactive tee junctions and can be used with various types of phase or time delay elements or components.
More particularly, a Butler matrix is a type of higher-level reciprocal orthogonal passive circuit characterized by an equal number of input ports and output ports, and equal power division of each input signal to the several output ports. The Butler matrix circuit provides equal power signal amplitudes delivered to each output port. A three-by-three Butler matrix includes three input ports and three output ports, a four-by-four Butler matrix includes four input ports and four output ports, an eight-by-eight Butler matrix includes eight input ports and eight output ports, and so forth. In addition, other well known circuits and circuit components can be constructed from hybrid junctions and other components such as phase shifters and resistors used for impedance matching and for analog signal processing. For example, monopulse comparators, diplexers, analog amplifiers, and beam steering circuits can be constructed in this manner. An important example of such a circuit is a high-level Butler matrix, which may be used to implement beam forming networks (BFNs) for multi-beam antenna systems with a large number of beams. These high-level Butler matrices may be constructed from complexes of four-by-four Butler matrices, which in turn may be constructed from complexes of hybrid junctions and other circuit elements, such as phase shifters.
For constructing high-level Butler matrices using hybrid junctions, see “Multiple Beams from Linear Arrays” by J. P. Shelton and K. S. Kelleher, published in the March 1961 “IRE Transactions on Antennas and Propagation.” For constructing monopulse comparators using hybrid junctions, see “A Wide-Band Monopulse Comparator With Complete Nulling in All Delta Channels Throughout Sum Channel Bandwidth” by Kian Sen Ang, Yoke Choy Leong and Chee How Lee, published in the February 2003 “IEEE Transactions on Microwave Theory and Techniques.” For constructing diplexer circuits using hybrid junctions, see “A Diplexer Using Hybrid Junctions” by Leon J. Ricardi published in the August 1966 “IEEE Transactions on Microwave Theory and Techniques.” For constructing hybrid matrix amplifiers (HMAs) using hybrid junctions, see “Multiport Power Amplifiers For Mobile-Radio Systems Using Microstrip Butler Matrices” by A. Angelucci, P. Audagnotto, P. Corda, and B. Piovano, published in the June 1994 Antennas and Propagation Society International Symposium. Those skilled in the art will appreciate that other devices, and in particular more complicated HMAs and beam forming networks for multi-beam antenna systems, may be constructed using the principles and techniques taught in this specification and in the documents referenced above, which are incorporated herein by reference.
In particular, hybrid junctions form the basic building blocks for the beam forming networks that are used in shaped beam and/or multi-beam antenna systems having a wide range of applications including, but not limited to, antennas for wireless telephone base stations, radars, missile guidance systems, missile defense systems, satellite surveillance systems, and satellite communication systems. In general, component beams may be pointed in different directions so to allow for substantially isolated input ports corresponding to each component beam. Component beams can be combined in various ways to form composite beams. Each beam may be encoded with beam-specific information and combining can occur for analog or digital signals.
To create these capabilities, the multi-beam antenna system includes a beam forming network or circuit that transfers signal energy from one or more input ports to a plurality of output ports operatively connected to one or more antenna elements to emit or receive the desired beams. Although the most critical design considerations may vary from application to application, it is generally desirable to manufacture beam forming networks that are inexpensive and easy to manufacture, repeatable in performance characteristics, light in weight, small in size, reliable and durable in construction, low in RF signal losses, low in noise generation, easy to ground properly, and easy to maintain. Although other design objectives may also be important in a particular application, this list includes many of the most important design considerations for many applications.
A number of these design objectives can be satisfied by manufacturing the signal processing circuits on printed circuit (PC) boards constructed from a dielectric substrate and using stripline carried on the dielectric substrate as the transmission media. The dielectric substrate typically has a ground plane attached to one side and the stripline carried on the other side. This configuration produces a circuit that can be mass produced on a PC board using conventional etching technology and processes. The resulting device exhibits low manufacturing costs, reliability, durability, repeatable performance characteristics, and accessible and solid ground characteristics. These circuits can be readily designed to exhibit low RF signal losses and low noise generation. The drawback in using this construction paradigm is that beam forming networks using hybrid junctions are often characterized by crossover points in which stripline segments must pass by each other physically without interfering with each other electrically.
On a PC board, the need for crossovers presents a design challenge because the stripline segments must remain physically separated from each other to avoid electrical interconnection (if the stripline segments physically touch each other) or radiating interference or cross-talk (if the stripline segments come too close together without physically touching each other). A number of techniques have been developed to implement crossovers for stripline signal processors implemented on PC boards, such as “flying bridge” sections of PC board that physically jump one stripline segment over another, coaxial cable links to cross each other, and multiple layered PC board constructs with conductors suspended in air and extending between PC boards to implement crossovers. Each of these designs increases the cost of the circuit, reduces the physical ruggedness of the circuit, and has the potential to increase noise generation and RF signal loss, particularly at junctions between different types of transmission media segments. More importantly, these somewhat clumsy solutions to the crossover problem greatly complicate the manufacturing process because the entire circuit cannot be arranged on a single PC board using stripline transmission media segments formed into the PC board that can then be manufactured through a conventional etching techniques and processes.
Another technique employs a circuit known as a “zero-dB crossover” that can be comprised of two cascaded quadrature hybrid junctions. Although this type of crossover can be implemented on a single flat PC board without physical trace jumps, it occupies a relatively large section of PC board space. Because the crossover is a basic building block that is repeated many times in creating a higher-level beam forming network, the significant board size required to implement the zero-dB crossover quickly multiplies into an overly large and expensive PC board as the complexity of the bream forming network increases.
In addition to the problem of crossovers, stripline signal processing circuits arranged on PC boards must maintain proper physical spacing between the stripline segments to avoid radiating interference. Further, designing each stripline segment to have a precisely determined phase characteristic at RF and microwave operational frequencies also requires the stripline circuit to be physically arranged on the printed circuit board in a manner that consumes a relatively large amount of planar board space. To maintain proper spacing and minimize the number of crossovers required, and to take advantage of the natural symmetry of the circuits, they are typically arranged out linearly, with the inputs ports spaced along one side and the output ports spaced along the other side of the stripline circuit. The transfer function of the stripline circuit then processes the signal as it propagates across the PC board from the input ports to the output ports.
For this type of circuit configuration operating at a carrier frequency of 1.92 GHz (which is the center frequency of the authorized PCS wireless telephone band), a conventional hybrid junction layout typically occupies PC board space that is approximately one quarter of a square wavelength “in the guide” (λg) (e.g., an approximately square section of PC board that is λg/2 in length on each side). A typical dielectric material (e.g., PTFE Teflon®) having a dielectric constant equal to 2.2 (∈r=2.2) can be used to construct PC boards that will exhibit an effective dielectric constant of 1.85 (∈reff=1.85) for microstrip transmission media segments exposed to the PC board on one side and exposed to air on the other side. For this type of PC board circuit, the wavelength in the guide (λg) (i.e., the wavelength as propagating in the stripline transmission media as laid out on the PC board with one side exposed to the dielectric substrate and the other side exposed to air) is approximately 4.52 inches (11.48 cm), which results in a side dimension of the PC board required to implement a quadrature hybrid junction of approximately 1.13 inches (2.87 cm). It is well known to someone familiar with the art that using a substrate material having a higher dielectric constant value can reduce the overall size of the circuit. Materials with substantially higher dielectric constant values can be substantially more expensive, can have higher RF signal losses, and can have RF power handling limitations that are a lower value due to reduced stripline trace width values. It is desirable to have a circuit with sufficiently wide conducting trace width values and low RF signal loss characteristics for conditions of moderate to high operational RF power levels. Generally, the use of a substrate material with a low dielectric constant value is often desirable when RF power levels are a significant design consideration.
Using this technology and connecting four hybrid junctions together to construct a four-by-four Butler matrix occupies PC board space that is approximately one square wavelength in the guide (λg), which at a carrier frequency of 1.92 GHz results in a side dimension of the PC board required to implement the four-by-four Butler matrix of at least 4.52 inches (11.48 cm) using microstrip on a dielectric material having a dielectric constant equal to 2.2 (∈r=2.2). The physical size of the PC board starts to become unwieldy and expensive as the number of hybrid junction elements increases beyond the eight to 16 element level. For example, a 64×64 Butler matrix requires 48 hybrid junctions and associated crossovers, and a 128×128 Butler matrix requires 160 hybrid junctions and associated crossovers. Arranging a stripline signal processing circuit on a planar PC board in the conventional manner for these circuits would result on a very large PC board that would be very expensive to manufacture and install in a secure manner.
An approach to solving some of the problems associated with PC board stripline signal processing circuit design is provided in Tanaka et al., U.S. Pat. No. 6,252,560, which describes a four-by-four Butler matrix that is arranged on a double-sided dielectric PC board with a ground plane located in the center. See Tanaka at FIG. 7. This allows the first stage hybrid junctions to be carried on a first side of the double-sided dielectric PC board, and the second stage hybrid junctions to be carried on the other side of the double-sided dielectric PC board. Crossovers are conveniently implemented using tap-through connections between the stripline circuits located on opposite sides of the PC board. This provides an elegant, low noise and space effective mechanism for implementing the crossovers. However, the circuit is still laid out linearly with the input ports located on the other side of the circuit from the output ports. In addition, the Tanaka reference shows the Butler matrix implemented on a common board with a power divider network feeding a set of patch radiators. Tanaka does not teach or suggest further steps to reduce the physical size of the Butler matrix circuit. Nor does it teach or suggest mechanisms for minimizing the PC board space required to implement higher-order analog signal processing circuits.
Accordingly, a continuing need exists for stripline signal processing networks that are inexpensive and easy to manufacture, repeatable in performance characteristics, light in weight, small in size, reliable and durable in construction, low in RF signal losses, low in noise generation, easy to ground properly, and easy to maintain. More specifically, a need exists for improvements in stripline signal processing circuit design that reduce the PC board space required to implement higher-order stripline signal processing circuits.